As dental elastic impression materials, those in which the stock is comprised of agar hydrocolloid, alginates, polysulfide rubbers, polyether rubbers, silicone rubbers, or the like are used. These elastic impression materials are suitable for impression taking of complicated forms having an undercut in the oral cavity, such as tooth roots, dentitions, jaws, and mucous membranes because the deformation generated upon being removed out from the oral cavity is quickly recovered.
Among these elastic impression materials, the agar hydrocolloid impression materials and the alginate impression materials have a suitable elasticity from the clinical viewpoints. On the other hand, however, these elastic materials are large in the permanent deformation having a high content of water in the structures thereof, and therefore, they have such properties that the water content is vaporized from the taken impressions, resulting in big changes in the dimension with a lapse of time. Furthermore, since these elastic materials have such drawbacks that they are likely to come off due to their low tear strengths, they are mainly used for preliminary impression taking.
On the other hand, the synthetic rubber-based elastic impression materials in which the stock is comprised of polysulfide rubbers, polyether rubbers, silicone rubbers, or the like have such advantages that the detail reproducibility is superior, the changes in the dimension are extremely small, and they hardly come off. For these reasons, these synthetic rubber-based elastic impression materials are mainly used for precision impression taking. However, the polysulfide-based impression materials in which the stock is comprised of polysulfide rubbers have such drawbacks that the unpleasant odor is strong and that the setting is slow. Also, the polyether rubber-based impression materials are superior in the hydrophilicity and convenient for taking impressions in the state that a saliva is present as in the oral cavity. However, the polyether rubbers-based impression materials have such a drawback that it involves difficulty to remove out the impression from the oral cavity of a patient, because the rubber elasticity is low, and the set material is hard. Furthermore, since the polyether rubbers have an inherent bitter taste, the polyether rubber-based impression materials have such a drawback that they give an unpleasant feeling to a patient. Moreover, since the polyether rubber-based impression materials have a large permanent deformation, there is a danger that the deformation occurs at the time of removing out the impression from the oral cavity.
In contrast thereto, the impression materials in which the stock is comprised of silicone rubbers include a condensation polymerization type and an addition polymerization type depending on the setting process thereof. Any of these room temperature-vulcanizing silicone rubbers are used as dental silicone impression materials. Of these materials, the addition polymerization type silicone impression materials are sharp in the setting, small in the permanent deformation and extremely small in the dimensional changes and hence, are most likely used as the precision impression. However, as compared with the alginate impression materials, the silicone rubber-based impression materials are so high in the hardness after the setting that when the impression is removed out from the oral cavity of a patient, they give the patient pain, and when a plaster model is prepared using the thus taken impression and then removed out from the impression, the plaster may sometimes be broken depending on the site. Also, as compared with the polyether rubber-based impression materials, the silicone rubber-based impression materials are inferior in the hydrophilicity that the detail impression may be unclear due to the saliva in the oral cavity. In addition, since the tear strength is not sufficiently high, when the impression is removed out from the oral cavity of a patient, the impression material may come off depending on the impression site, resulting in making the impression incomplete.
In the light of the above, while the currently used impression materials have inherent advantages, they involve various disadvantages. Any of these impression materials were incomplete as impression materials for reproducing the state in the oral cavity in detail and preparing dental prostheses with a good fitness accuracy, without giving a patient pain.
Of the above-described respective impression materials, the present inventors paid attention to the addition polymerization type silicone impression materials which are not only small in the dimensional changes and permanent deformation but also superior in obtaining that an accurate impression can be taken. Thus, an object of the present invention is to develop a dental impression silicone composition provided with excellent properties for the precision impression by improving such defects of the addition polymerization type silicone that the hardness after the setting is so high that the strain in compression is small, the wetting against the water content such as saliva is poor and that the tear strength is not sufficient, while enhancing the superior characteristics thereof.